Molded products obtained from carbon fibers, a cement, and water have been developed in an attempt to reinforce composites comprising a cured cement composition as a matrix, such as mortar and concrete, by taking advantage of the tensile strength and elasticity of carbon fibres. These molded products have been conventionally obtained by mixing short carbon fibers with a cement and water, and an additional aggregate and/or various admixtures for cement, if desired, and curing the resulting composition.
Since carbon fibers are readily broken during kneading due to their high modulus of elasticity and poor flexing characteristics, improvements have been proposed to prevent breakage by using aggregates of a spherical shape, incorporating air or polymers to the system, and the like. However, the amount of carbon fibers which can be incorporated according to these proposals is only about 5% by volume at the most. Several improvements on these techniques have been reported. For example, the use of cement having a maximum particle size of 45 .mu.m or smaller produces good results, or addition of ultrafine silica having latent hydraulic properties and having an average particle size of about 0.1 .mu.m to common portland cement produces good results. However, sufficient attention has not been paid to the fact that carbon fibers have poor resistance to flexing, so that the straightness of carbon fibers and uniform dispersion of the carbon fibers in the cured cement matrix have not been assured, and the reinforcing effect of carbon fibers has been insufficient. Further, the addition of ultrafine powders of silica is intended to fill spaces among cement particles or between fibers and cement particles, thereby to ensure adhesion between the fibers and the matrix. However, the reaction of the ultrafine silica is a Pozzolanic reaction and the adhesive force produced thereby is limited and insufficient.
Methods of impregnating a cement composition into continuous fibers include the techniques known for molding glass fiber-reinforced plastics, such as hand lay-up, filament winding, pull-pressing, etc. According to these techniques, common portland cement or portland cement having a slightly smaller particle size (e.g., an average particle diameter of 20 .mu.m) is used without special attention to the particle size, As a result, the amount of carbon fibers which can be incorporated is about 10% by volume at the highest. Besides, although the carbon fibers incorporated by these methods are unidirectionally oriented, microscopic straightness of the carbon fibers and dispersibility of individual fibers are not assured so that the reinforcing effect of the carbon fibers is incomplete. In addition, the cement particles are not sufficiently embedded in gaps between carbon fibers because they have a large average particle size as described above, thus resulting in insufficient binding between the carbon fibers and the cement.
Another proposal suggests using carbon fibers in the form of a mat or cloth and molding with a cement and various additives for cement. This technique involves similar problems to those described above and, also, the amount of the carbon fibers to be incorporated thereby does not exceed 5% by volume. Therefore, the strength of the resulting composite material is limited.